The invention relates to a method for detecting the aging of a heterogeneous catalytic converter, to an exhaust gas after-treatment system for an internal combustion engine and to an internal combustion engine.
Heterogeneous catalytic converters, that is to say catalytic converters whose material is present in a phase or an aggregate state which is different from a phase or an aggregate state of a medium which is to be converted at the catalytic converter have, as an essential functional principle, adsorption or storage of at least one of the components which are involved in a reaction at the catalytic converter. The absorption capacity or storage capacity is subject to aging, which is typically unpredictable, or at any rate virtually unpredictable. In this context, various mechanisms contribute to the aging of the catalytic converter. For example, contamination is possible in which substances which are not involved in the desired reaction adhere permanently to the surface of the catalyst and thereby block binding sites for the surfaces involved in the reaction. Thermal aging is also possible, in particular in that surface regions of a solid catalyst material sinter together to form relatively large particles, with the result that the surface which is available for the catalyst becomes smaller. A change in the basic chemical structure of a catalytic material is also possible over the course of time, wherein binding energy levels at the surface change. This can, on the one hand, lead to a situation in which reactants are bound to the surface to a smaller degree, and it can also lead to a situation in which the binding energy levels are increased so strongly that reactants which have been bound can no longer react and remain bound. As a result, binding locations are in turn occupied and are no longer available for the further catalyst. In particular, owing to the large number of possible aging mechanisms, and the various processes which occur in this context it is virtually impossible to predict the actual aging of a specific catalytic converter. However, it is apparent that modeling of the catalytic reaction requires the storage capacity as an output variable. Such modeling is carried out, in particular in order to be able to operate an internal combustion engine while complying with legal emission limiting values. The modeling is used here, in particular, to regulate an exhaust gas after-treatment system of the internal combustion engine and/or the internal combustion engine itself.
Within the scope of known methods for modeling the catalytic reaction of the catalytic converter, a storage capacity is estimated or assumed to be constant. Aging of the catalytic converter is typically included in the calculations here as a global deterioration factor, which is, under certain circumstances defined with a predetermined time dependence. German laid-open patent application DE 10 2011 114 700 A1 discloses a method for detecting the aging of a catalytic converter, but it depends on a global decline in the conversion rate at the catalytic converter. The storage capacity cannot be detected directly using this method.